DESIGN OF CASTINGS - Concordia Universityusers.encs.concordia.ca/~nrskumar/Index_files/Mech423/Lecture 11.pdfLecture 10 1 • Brazing-type ... • Important steps in making a good

1 Lecture 10

• Brazing-type operation where filler metal melting point is below

450oC (840oF). Typically used for connecting thin metals, electronic

components (mostly where higher temperature should be avoided)

• Important steps in making a good soldered joint:

• design of acceptable joint

• selection of correct solder metal for the job

• selection of proper flux

• cleaning surfaces

• application of flux, solder and heat to fill joint by capillary action

• removing residual flux if required.

Soldering

2 Lecture 10

• Used for wide variety of sizes, shapes and thickness joints. (clearance)

• Extensively used for electrical couplings and gas/air-tight seals.

• Shear strength is usually less than 2MPa. So if more strength required

usually combined with other form of mechanical joint as seam-lock.

Solder Joint Design

• Avoid butt joints, and soldering

where joint is subject to peeling.

• Parts need to be held firmly until

solder is completely solidified.

• Flux should be removed after

soldering (method depends on

type of flux; water, alcohol etc.).

3 Lecture 10

Metals to be Joined

• Copper, silver, gold, tin plated steels easily joined

• Aluminum (has strong oxide film) so difficult to solder unless using

special fluxes and modified techniques (used in automotive radiators)

4 Lecture 10

• Usually low MP alloys Lead-Tin alloys (+ antimony 0.5%)

• low cost, reasonable mechanical properties.

• Good knowledge base

• plumbing, electronics, car-body dent repair, radiators.

• Tin is more expensive than lead, so lower tin compositions used

unless lower melting point, higher strength, higher fluidity required.

• High melting point – higher lead content (cheaper)

• “Mushy” wiping solder has 30-40% tin.

• Low melting point solder has eutectic composition (62%Sn -

38%Pb) fast melting, fast freezing, high strength.

Solder Metals

5 Lecture 10

Solder Metals • Lead-free solders - Used where

lead toxicity may be a problem.

(water supplies etc).

• Other alloys include

• Tin-antimony (higher

melting points)

• Bismuth

• Tin-indium

6 Lecture 10

• Same principles as brazing so surfaces must be clean; mechanical or

chemical cleaning.

• Fluxes remove surface oxides:

• Corrosive: muriatic acid, zinc/ammonium chlorides. Al, steels, copper,

brass, bronze….

• Non-corrosive: rosin (residue after distilling turpentine), good for

copper, brass, tin or silver-plated surfaces

• Heating for Soldering

• Similar to brazing, (furnace and salt bath heating is not usually used)

• Wave soldering is used for wires while dip soldering for auto parts

• Hand soldering is done by solder iron and oxy fuel torch

Soldering Fluxes

7 Lecture 11

Soldering Heat

8 Lecture 11

Lecture 11

Adhesive & Mechanical Fastening

MECH 423 Casting, Welding, Heat

Treating and NDT

Credits: 3.5 Session: Fall

Time: _ _ W _ F 14:45 - 16:00

9 Lecture 11

Adhesive Bonding

10 Lecture 11

• Inexpensive and weigh less than fasteners for similar strength joints

• provide thermal and electrical insulation; act as a damper to noise,

shock, and vibration; stop a propagating crack; and provide protection

against galvanic corrosion when dissimilar metals are joined

• Seal against moisture, gases, and fluids; Offer corrosion resistance

• Most adhesives can be applied quickly, and useful strengths are

achieved in a short period of time. (2 seconds!)

• Surface preparation may be reduced - bonding can occur over an

oxide film, and rough surfaces are good - increased contact area.

• Tolerances are less critical since the adhesives are more forgiving

than alternative methods of bonding

• Smooth contours are obtainable, (no holes required cf. bolts).

Adhesive Bonding

11 Lecture 11

• Biggest advantage is in load distribution, unlike mechanical fasteners

• Non metallic (adhesive) material is used to join surfaces

• Can be thermoplastic or thermosetting resins, elastomers, ceramics…

• Applied as drops, beads, pellets, tapes, coatings (liquids, pastes…)

• Curing can be influenced by heating, radiation or light, catalyst or

chemical reactions

• Different applications of adhesive fastening

• Heavy duty or Load bearing (structural adhesive)

• Light duty or fixturing (mainly to hold pieces together)

• Sealing (forming liquid or gas-tight joints)

• Due to load bearing nature of structural adhesive, strength and rigidity

of the adhesive material over a period of time is important

Adhesive Materials and Properties

12 Lecture 11

1. EPOXY – oldest and more common (single or two component) single

component needs heat to cure, two component cures at RT

2. CYANOACRYLATES - single component cures at RT, moisture promotes

curing < 2s

3. ANAEROBICS - single component, remains liquid in air, shut of O in joints,

cures by polymerizing in the presence of Fe or Cu. 6 to 24 hrs to attain useful

strength

4. ACRYLICS - two component can be applied and stored separately. Once

joined, they cures at RT. thermoplastic

5. URETHANE – used in low temp service <65°C

6. SILICONE – resistant yet flexible joint. Cures from moisture on surface

7. HIGH-TEMPERATURE ADHESIVES – expensive as silicone, good for use

clost to 300°C. slow curing. Aerospace applications


21 Lecture 11

8. HOT MELTS - not considered to be true structural adhesives, but are

being used to transmit loads, mostly in composite mat’l assemblies

• They are thermoplastic resins - solid at room temperature but melt

abruptly when heated into the range of 200 to 300°F (l00 to 150°C)

• Applied as heated liquids to form bond as the molten adhesive cools

• Or position the adhesive in the joint prior to operations (paint bake

process) in automobile manufacture. while baking, the adhesive

melts, flows into seams, and seals against corrosive moisture entry

• The hot melts provide reasonable strength within minutes, but do

soften and creep when exposed to elevated temperatures and

become brittle when cold.


22 Lecture 11


23 Lecture 11

1. What materials are being joined? What are their porosity, hardness, &

surface conditions? Difference in thermal expansions (contractions)

2. How will the joined assembly be used? What type of joint is proposed,

what will be the bond area, and what will be the applied stresses? How

much strength is required? Will there be mechanical vibration,

acoustical vibration, or impacts?

3. What temperatures might be required to effect the cure, and what

temperatures might be encountered during service? (highest

temperature, lowest temperature, rates of temperature change,

frequency, duration of exposure to extremes, properties required, and

differential expansions or contractions.)

Design Considerations

24 Lecture 11

4. Will there be subsequent exposure to solvents,

water or humidity, fuels or oils, light, ultraviolet

radiation, acid solutions, or general weathering?

5. What is the desired level of flexibility or stiffness?

How much toughness is required?

6. Over what length of time is stability desired? What

portion of this time will be under load?

7. Is appearance important?

8. How will the adhesive be applied? What equipment,

labor, and skill are required?

9. What will it cost?


25 Lecture 11


26 Lecture 11

1. Most material/combination can be joined (size, shape, and thickness).

2. For most adhesives, low curing temperatures, (usually <180°C). Heat-

sensitive materials can be joined without damage and HAZ.

3. Foils can be joined to each other or to heavier sections. When joining

dissimilar materials, the adhesive provides a bond that can tolerate the

stresses of differential expansion and contraction.

4. Adhesives bond the entire joint area good load distribution and

fatigue resistance; stress concentrations are avoided (unlike screws).

Total joint strength comparable with alternative methods. (Shear

strengths of industrial adhesives > 20MPa or 300 PSI)

5. Additives can enhance strength, increase flexibility, provide resistance

to various environments.

Adhesive Bonding Advantages

27 Lecture 11

1. No universal adhesive. Selection is complicated by available options.

2. Most adhesives are not stable above 350°F (180°C). (Max 260°C).

3. High-strength adhesives are often brittle (poor impact properties).

Resilient ones often creep. Some become brittle at low temperatures

4. Long-term durability and life expectancy are difficult to predict.

5. Surface preparation and cleanliness, adhesive preparation, and curing

can be critical for consistent results. Adhesives are sensitive to grease,

oil, moisture. Surface roughness and wetting must be controlled.

6. Difficult to determine the quality by traditional NDT techniques.

7. Adhesives contain toxic chemicals/solvents, or produce upon curing.

8. Many structural adhesives deteriorate under certain operating

conditions, UV light, ozone, acid rain, moisture, and salt.

9. Adhesively bonded joints cannot be readily disassembled.

Adhesive Bonding Disadvantages

28 Lecture 11

Mechanical Fastening

29 Lecture 11

• Often based on localized, point-attachment processes, in which the

join is provided by a nail, a rivet, a screw or a bolt.

• These joints depend on residual tensile stresses in the attachment

to hold the components in compression.

• The joint is usually formed by an ordered array of point-

attachments, rivets at the edge of a ship's plate, or the uniformly

spaced bolts around a pressure vessel flange.

• Mechanical joints are also made along a line of attachment, such

as that formed when a piece of sheet is bent to form a cylinder (a

paint can, for example) and the two edges are joined with an

interlock seam.

Mechanical Joining Methods

30 Lecture 11

• Here too the residual stresses (tensile around the circumference of

the can, compressive along the joint) ensure integrity of joint.

• Many mechanical joints are designed for ease of assembly and

disassembly (for example, bolted joints).

• The effectiveness of mechanical fasteners depends upon

• The material of the fastener,

• Fastener design (including the load-bearing area of the head)

• Hole preparation, The installation procedure.

• The desire is to achieve uniform load transfer, minimum stress

concentration, and uniformity of installation torque or interference fit.

Mechanical Fasteners

31 Lecture 11

• Integral fasteners: formed areas of a

component that interfere or interlock with other

components - most commonly found in sheet

metal products: lanced/shear-formed tabs,

extruded hole flanges, embossed protrusions,

edge seams, and crimps.

• e.g. beverage can lids, tabs etc.


32 Lecture 11

• Discrete fasteners: separate pieces used

to join primary components: bolts, nuts,

screws, nails, rivets, quick-release

fasteners, staples, and wire stitches.

• Over 150 billion discrete fasteners - annual

consumption in the US, 27 billion by auto

industry. (ANSI BI8.12.) The fasteners are

easy to install, remove, and replace.

• Various finishes and coatings can be

applied to withstand a multitude of service

conditions.


33 Lecture 11

• Shrink and expansion fits: dimensional change introduced to one

or both of the components by heating or cooling (heating one part,

heating one and cooling other, or cooling one).

• Assembled and a strong interference fit is established when

temperature uniformity is restored.

• Joint strength is exceptionally high. Can be used to produce a pre-

stressed condition in a weak material; to replace costly stronger one.

• Similarly, a corrosion-resistant cladding or lining can be easily

provided to a less-costly bulk material.

• Press fits: similar to above but using mechanical force.


34 Lecture 11

1. Ease of disassembly and reassembly. (threaded fasteners) and

semipermanent fasteners (such as rivets) can be drilled out.

2. The ability to join similar or different materials in different sizes,

shapes, and joint designs. Some joint designs, such as hinges and

slides, permit limited motion between the components.

3. Low manufacturing cost compared to the components being joined.

They are readily available in a variety of mass-produced sizes.

4. Installation does not adversely affect the base materials as with

techniques involving the application of heat and/or pressure.

5. Little or no surface preparation or cleaning is required.

Mechanical Fasteners - Advantages

35 Lecture 11

• Mechanical joints use compressive residual stresses across the

join in order to maintain the components in contact.

• They therefore require a balancing tensile stress elsewhere in

the system.

• These tensile stresses may either be in the fastening (nails, bolts

or rivets), or in the components themselves (the interlock seam).

Role of Residual Stress

36 Lecture 11

• Often require aligned holes for mechanical fasteners. Many ways

to make holes; drilling, punching, chemical machining, lasers etc.

• Each produces holes with characteristic features; surface finish,

tolerance, properties etc.

• If bolts need nuts then two-sided access is required (if threaded

hole – one side only).

• Self-tapping screws (harder screws or softer materials).

• Stapling is quick, cheap method that does not require prior hole.

• Rivets give good strength but are semi-permanent.

• Snap fits require material is sufficiently elastic.

Manufacturing Concerns

37 Lecture 11

• Must consider many factors including possible joint failure as

fasteners are vulnerable sites. Joints usually fail due to oversight in:

• Design of fastener and its method of manufacture.

• Material used for fastener. Joint design.

• Means and details of installation.

• E.g. Insufficient strength or corrosion resistance, (galvanic corrosion is

common with dissimilar metal fasteners). Non-metallic fasteners

(nylon, fiberglass) can sometimes be used at low stresses.

• 90% of cracks in airframes originate at fastener holes (stress raisers)

and fasteners fail by fatigue.

Design and Selection

38 Lecture 11

• Problems worsen if

too loose or too tight.

• Rolled threads and

formed heads are

better than machined

and fillets are

important between

head and shank of

bolts.

Design and Selection

39 Lecture 11

Lecture 11

Welding Joints & Metallurgy

MECH 423 Casting, Welding, Heat

Treating and NDT

Credits: 3.5 Session: Fall

Time: _ T _ _ _17:45 - 20:15

40 Lecture 11

Flow of Heat in Welds

• Heat (energy) is introduced into workpiece to cause melting during

fusion welding. Not all heat contributes to melting. Some conducted

away raising temperature of surrounding material causing

(unwanted) metallurgical & geometrical changes - AKA – HAZ.

• How the heat is distributed directly influences:

• the rate and extent of melting; (affects weld volume, shape,

homogeneity, shrinkage, distortion, related defects).

• the rate of cooling and solidification; (solidification structure,

related properties).

• the rate of heating and cooling in the HAZ; (thermally induced

stresses, cooling rate in solidification zone, structural changes in

HAZ, distortion, residual stresses).

41 Lecture 11

• A fusion weld produces several distinct microstructural zones in

both pure metals and alloys.

• Fusion zone, FZ: – portion of metal that is melted during welding

(above Tm or TL for alloy).

• Partially Melted Zone, PMZ: – for an alloy where temperature is

between TLiquidus and TSolidus. (No PMZ in pure metal).

• Heat Affected Zone HAZ: – portion of base material that was not

melted but whose properties are affected by heat of welding

(phase transformation, reaction).

• Unaffected Base Material UBM: – portion of base material which

has not been affected by welding heat.

Weld Zones Prediction

42 Lecture 11

Schematic of the distinct zones in a fusion weld in a pure metal (a) and an alloy (c) as these correspond to phase regions in the hypothetical phase diagram shown (b).

The various microstructural zones formed in fusion welds between a pure metal (right) and an alloy (alloy).

Weld Zones Prediction

43 Lecture 11

Simplified welding equations.

Peak Temperatures in solid metal:

0

5.0

0

121

TTH

Chye

TT mnetP

where:

T0 = temperature of workpiece at start of welding (K)

TP = Peak temperature at distance y from fusion boundary (K)

Tm = melting temperature (or liquidus) of metal being welded (K)

= density of metal (g.m-3)

C = specific heat (J.g-1 .K-1)

Hnet = heat input (J.m-1) = q/v = EI/v for arc welding processes

h = thickness of base material (m)

e = base of natural logarithms (2.718)

y = distance form fusion zone (= 0 at the fusion zone, where TP = Tm) (m)

44 Lecture 11

Solidification rate

The rate at which weld metal solidifies can have a strong effect

on microstructure and properties.

Solidification time, St , in seconds:

202 TTCk

LHS

m

nett

where:

L = Latent heat of fusion (J/m3)

T0 = temperature of workpiece at start of welding (K)

Tm = melting temperature (or liquidus) of metal being welded (K)

k = thermal conductivity (J.m-1.s-1. K-1)




45 Lecture 11

Cooling Rates

Final metallurgical state of FZ and HAZ is primarily determined

by cooling rates. Affects fineness/coarseness of grains,

homogeneity, phases, microconstituents etc. Especially in

steels where some phase transformations are dependent on

cooling rate (fast cooling can produce hard, brittle martensite).

For a single pass in a butt joint between thick plates (> 6

passes) of equal thickness:

net

C

H

TTkR

2

02

where:

R = cooling rate at the weld centreline (K/s)

T0 = initial temperature of workpiece (K)

TC = temperature at which cooling rate is calculated (K)



46 Lecture 11

For thin plates ( < 4 passes):

0

2

2 TTH

hCkR C

net

where:

R = cooling rate at the weld centreline (K/s)

T0 = initial temperature of workpiece (K)

TC = temperature at which cooling rate is calculated (K)




C = volumetric specific heat (J.m-1 .K-1)


Note: increasing the initial temperature, T0, (by preheating)

decreases the cooling rate,R.

47 Lecture 11

Schematic of the effect of weldment and weld geometry on the

dimensionality of heat flow: (a) two-dimensional heat flow for full-

penetration welds in thin plates or sheets; (b) two-dimensional heat

flow for full-penetration welds with parallel sides (as in EBW and some LBW); (c) three-dimensional heat flow for partial- penetration

welds in thick plate; and (d) an intermediate, 2.5-D condition for

near-full-penetration welds.

48 Lecture 11

• Heat flow in weld is affected by size and shape of weld.

• Surfacing or Bead welds, made directly, no surface

preparation. Used for joining thin sheets, adding

coatings over surfaces (wear resistance)

• Groove Welds – full thickness strength, done as V,

Double V u, and J (one side prepared). The type of

groove depends on the thickness of the joint, weld

process and position

Fundamental types of welds, including (a) groove, (b) fillet, (c) plug,

and (d) surfacing.

Weld Joint Configuration

49 Lecture 11

• Fillet Welds, used for tee, lap or corner joints. No edge

preparation. Size of the weld is measured by the

largest 45° right triangle that could be drawn in the

weld cross section.

• Plug Weld – attach one part over another replacing

rivets are bolts. Normally a hole is made on the top

plate and welding done at the bottom of the hole

• Five basic weld designs and some typical joints are

shown in the figure

Fundamental types of welds, including (a) groove, (b) fillet, (c) plug,

and (d) surfacing.


50 Lecture 11


• Inserts are used

in pipelines or

other places

where welding

is restricted to

one side only

51 Lecture 11

Five basic weld designs:

(a) butt, (b) corner, (c)

edge, (d) lap, (e) tee.

Some typical weld joint variations.


• Type of loading will

decide the type of

joint, to prevent

failure

• Accessibility and

cost are other

considerations

• Cost is affected by the amount of weld

metal, type of weld equipment, speed

and ease of welding

52 Lecture 11

(a) Single V, (b) double

V, (c) single U, (d)

double U joints. Require

filler metal.

(a) Full penetration,

(b) partial

penetration, (c)

continuous, (d)

intermittent welds.


53 Lecture 11


54 Lecture 11

• Straight butt joints do not require filler metal as long as faces

abut tightly (gaps less than 1.5 mm) usually requires machined

surface (not sawn) – GTAW, PAW, LBW, EBW .

• Other joint configurations (V, double V, J, U etc) require filler

metal and preparation is made by cutting, machining etc. –

SMAW, FCAW, GMAW, SAW.

• Likewise with corner and edge joints. Some can be done

without preparation, others require machining.


55 Lecture 11

• Welding is a unique process producing

monolithic structures (one-piece from 2 or

more pieces welded together)

• If pieces joined together, and if there is a

crack in one, it does not propagate to other

piece normally.

Weld Design Considerations

• In case of welding, since it becomes single piece, crack can

propagate through to other piece. (The crack can initiate in the weld

or otherwise). - reflects the monolithic nature of welding process.

• Another consideration is small pieces may behave differently

compared to larger pieces of steel (shown in figure)

56 Lecture 11

• Joint designed primarily for load-carrying ability.

• Variable in design and layout can affect costs, distortion, reliability,

inspection, corrosion, type of defects.

• Select design that requires least amount of weld metal. (minimizes

distortion, residual stresses).

1. Where possible use square grooves (cheaper) and partial

penetration (helps maintain dimensions – unmelted metal in

contact) except where stress raisers cannot be tolerated (fatigue).

2. Use lap and fillet (instead of groove) welds where fatigue is not a

problem (cheaper).


57 Lecture 11

3. Use double-V double-U (instead of single-V or –U) for thick plates

(reduces weld metal vol.; controls distortion & balances heat input).

4. For corner joints in thick plates where fillet welds are inadequate,

bevel both plates to reduce tendency for lamellar tearing.


5. Design so weld can be accessed and inspected.

6. Over designing is a common problem in welding

that should be avoided (causes excessive weight

and costs – as a fillet weld side increases x2 the

weld metal increases by x4

58 Lecture 11

Weld Metallurgy

• Remember(?)

• HEAT TREATMENT and how various microstructures + properties

can be obtained by different cooling rates.

• CASTING - liquids shrink on solidifying, type of

grain structures, segregation, etc.

• WELDING - combines both usually:

• Melting + solidifying of weld pool

• Varying heating/cooling rates

59 Lecture 11

Weld Metallurgy

• Figure shows a welding where Metals A and B are welded with

Metal C as a backing plate and Metal D as a filler

• Molten pool is a complex alloy of ABCD held in

place by metal mould (formed by solids)

• Fusion welding can be viewed as a casting with

small amount of molten metal

• Resultant structure can be

understood if it is analyzed as casting and

subsequent heat treating

60 Lecture 11

• The composition of the material in the weld pool depends on the

joint design

• Upper design has more base and lower one has more filler metal

• Microstructure in this zone depends purely on the cooling rate of

the metal as in casting

Weld Fusion Zone

• This region cannot have properties similar

to that of the wrought parent metal

• Mainly because casting is inferior to

wrought products and metal in the fusion

zone has solidified from molten state as in

casting

61 Lecture 11

• All of these can affect microstructure

• Heating up to welding temperature

• Cooling down from welding temperature

• Holding at temperature during welding

• Formation of molten metal

• Solidification of molten metal

• As weld can be considered as a mini-“casting”:

• cast metal is always inferior to same alloy in wrought

condition.

• Good mechanical properties can be attained only if the filler

metal has properties (in as deposited condition) superior to or

equal to that of parent wrought metal

Weld Fusion Zone

Manual arc multi-pass welds of

(a) single vee-butt and (b)

double vee-butt weld. Plate is

180mm (7”) thick!

62 Lecture 11

• So may use filler metal/electrode of slightly different

composition.

• Structure is changed (due to melting and solidification in short time

due to low volume of molten metal ).

• Fusion zone is “casting”. Cooling rates influence grain structure

• Variation in grain structure, gas porosity, shrinkage, cracks and

similar to that of casting

• Contributing factors include: impurities, base metal dilution of filler,

turbulence & mixing, “casting” and “mould” interact, large

temperature gradients, dynamic (moving) process etc.

Weld Fusion Zone

63 Lecture 11

Weld Fusion Zone

64 Lecture 11

• Adjacent to Fusion zone is region where temperature is not

sufficient to cause melting but is often high enough to change the

microstructure. (an abnormal, widely varying heat treatment).

• Phase transformations

• recrystallisation

• grain growth

• precipitation/coarsening

• Embrittlement, cracking

• Steels can get anywhere from brittle martensite to coarse pearlite.

• Usually HAZ is weakest region in material (especially if base

material is cold-worked or precipitation hardened).

Heat Affected Zone - HAZ

65 Lecture 11


• Altered structure – so no longer have positives of parent metal

• Not molten – cannot assume properties of solidified weld metal

• Making this the weakest zone in the weld

If there are no

obvious defects

like cracks in

the weld zone,

normally the

weld starts to

fail in HAZ

66 Lecture 12

• Grain structure

• grain structure in weld depends on cooling rate, type of

metal, shape of weld etc. Can be

• coarse, fine, equiaxed, dendritic.

• Electrodes “designed” to give fine equiaxed grains

but depends on volume of weld and cooling rate.

• Other casting defects may be present:

• entrapped gases

• segregation

• grain-size variation

• orientation variation


67 Lecture 12


Structure

varies based

on the

temperature

and the alloy

composition

68 Lecture 12


• Thermal characteristics of process have different HAZ

• Low heat input – high heat in metal, slower cooling and more HAZ

resultant structures are ductile (low strength and hardness)

• High heat input – low heat in metal, faster cooling and less HAZ

Base metal

thickness and

thermal

conductivity

also have

effect on HAZ

69 Lecture 12

• Control thermal characteristics of weld:

• Low rates of heat input (slow heating) large HAZ

• high input rate (fast heating) - small HAZ (fast cooling)

• HAZ increases as

• initial temperature increases

• welding speed decreases

• thermal conductivity of base metal increases

• base metal thickness decreases

• Geometry affects HAZ

• Fillet weld has smaller HAZ than Butt weld


70 Lecture 12


71 Lecture 12


• If as weld quality is not acceptable, heat treatment after

welding is done

• Micro structure variations can be reduced or eliminated but

the results will be restricted to those that can be achieved by

heat treatment

• Cold working conditions cannot be achieved

• Another major problem is the controlled heating and

cooling of large structures.

• Complex structures are produced by welding and there

are not many quench tanks or furnaces that could

accommodate these

72 Lecture 12


73 Lecture 12


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74 Lecture 12

• Reduce gradient in microstructural change by pre-heating

• reduces cooling rate in weld and HAZ. Less stress raisers -

Cu and Al (high thermal conductivity)

• For steels with >0.3%C normal welding may cause untempered

martensite (also in alloy steels with increased hardenability)

• pre- and post-weld heat treatments

• Low carbon, low alloy steels great for welding!!!!!

• Brazing and soldering don’t cause melting of base metal but

can get HAZ depending on metal/system.

• ALSO can get interdiffsuion between filler and base metal to

form intermetallic compounds (these can add strength but are

often brittle)


75 Lecture 12

• Thermally-induced stresses

• usually produced in fusion welding these cause dimensional

changes, distortion and/or cracking

• Residual welding stresses due to:

• restraint (by rest of component) to thermal

expansion/contraction on heating/cooling

• weld is often in residual tension and base metal away from

weld is in residual compression

• Reaction stresses are induced when plates are restrained from

movement (clamped) these are additional stresses so

clamping has to be done very carefully - hence jig design.

Residual Stresses

76 Lecture 12

• As the weld is made, the liquid region

conforms to mould shape and adjacent

mat’l expands due to heat

• Weld pool can absorb expansion 90 to it,

but parallel is stopped by metal that is

cool

Residual Stresses

• So metal becomes thicker instead of longer causing a week zone

• Similarly, FZ & HAZ contracts while it is restricted by cooler UBM

• So it remains in a stretched condition, called “residual tension”

• This contracting region squeezes adjacent material producing

“residual compression”

77 Lecture 12

Residual Stresses

78 Lecture 12

• Presence of stress concentrators is very harmful:

• notches

• sharp interior corners

• cracks

• gas pockets

• slag pockets

• rough beads

• “strikes”

• ALSO restraint of base metal especially in heavy sections can be

very serious.

• Hence adherence to welding codes/practice is important and

also good weld design.

Residual Stresses

79 Lecture 12

• Common result of thermal stresses induced by

welding is distortion or warping of the assembly

• Various distortions occur in welding depending on

various weld configurations

• There are no fixed rules to avoid these distortions

• However, there are some general guidelines to

reduce these distortions

Distortion

80 Lecture 12

• Reduce heat input in to the weld

• Minimise volume of weld metal needed to form a joint

• Faster welding usually better (reduces welding time, as well

as volume of metal that is heated)

• Design weld sequences to have as few weld passes as

possible

• Allow base material as much freedom of movement

• Multiweld assemblies should be welded towards point of

greatest freedom from center to the edge

Reducing Distortion

81 Lecture 12

• Initial position can be disoriented to compensate for distortion and

get to desired final shape

• Restrain components completely so that plastic deformation occurs

in weld/material - (good for small weldments in relatively ductile

materials that do not crack)

• Stagger welds (eg. alternate sides of plate)

• Peen the weld and introduce compressive stresses

• Stress relief heat treatment prior to machining which may

unbalance residual stresses.

Reducing Distortion

82 Lecture 12

• Joint design is complex (to keep restraints to minimum so as to

prevent distortion/warping and cracking

• Selection of metal alloys (structure) with welding in mind and

special consideration given to welding thicker materials

• groove required to get access to root of joint

• minimum weld metal - maximum properties (J- and U- joints best

- minimum weld metal - more expensive to prepare)

• Minimum HAZ

• Proper size and shape of the weld bead reduces cracking

Reducing Cracking

83 Lecture 12

• Weld beads with high penetration (depth/width) are more prone to

cracking

• Reduce stresses by making cooling uniform or relaxing them by

promoting plasticity in weld metals

• Preheat and additional heating between weld passes to retard

cooling

• Some weld codes require thermal stress relief after weld before use

• Dissolved H2 in metal, electrode causes cracking, baking electrodes

or using low H2 electrodes are good as well to prevent cracking

Reducing Cracking

84 Lecture 12

Weldability and Joinability

Documents

DESIGN OF CASTINGS - Concordia Universityusers.encs.concordia.ca/~nrskumar/Index_files/Mech423/Lecture 11.pdfLecture 10 1 • Brazing-type ... • Important steps in making a good